The present invention is directed to an axle for retaining rollers within friction reducing fittings used in downhole applications and a method of installing the same. More particularly, but not exclusively, the present invention relates to an axle which is deformed at one or each end to retain the axle.
In downhole applications it is important that tools do not fail or, if they do fail, that they do not break in such a manner that parts of the tool are introduced into the well. If the axle supporting a roller fails this can result in the roller and axle being released into the well. In the case of a drilling application this can damage the drill bit or jam the drilling rig in place. Even if this does not happen, the friction reduction will be greatly diminished.
It is thus an object of the invention to provide a friction reducing fitting and an axle and method of installation that is simple and minimises the risk of axle failure or which at least provides the public with a useful choice. According to a first aspect of the invention there is provided a friction reducing fitting for downhole applications comprising:
a body portion for securement about a tubular member, the body portion having a pair of apertures;
a roller; and
an axle passing through said roller and said apertures in the body at either end of said roller, wherein the axle is deformed at at least one end to prevent movement of that end of the axle through the aperture in the body.
It has been found that surprisingly the axle can absorb the shock of a high impact load transmitted through the axle by deforming plastically. This means once the load is removed the axle and roller may revert to their original positions and remain secured to the fittings.
Conventionally, axles of the form employed in the present invention are hardened and thus are too brittle to be deformed to any great extent. Therefore, on first consideration, an axle having a deformed end would not be thought to be strong enough. However, it has been found that by only employing a small amount of deformation, an axle of sufficient strength can be formed.
The deformed end of the axle preferably has an enlarged diameter. This enables the deformed end of the axle to be conveniently enlarged by inserting a formation into a cavity formed in the end of the axle. Preferably the cavity and/or the formation are tapered to enable easy deformation.
In one embodiment the axle is deformed at both ends. In an alternative embodiment the axle is deformed at one end only and an enlarged diameter section is preformed at the other end of the axle.
The roller may be employed to reduce friction between the fitting and an internal component passing through the centre of the tubular member. However, the roller is typically located on the exterior of the body portion whereby the roller reduces drag in use between the fitting and a bore hole wall. In this case the invention prevents or at least reduces the chances of the roller being released into the bore hole.
The apertures in the body portion may be formed in a pair of ears extending from the body portion. Alternatively the body portion may have a cavity formed therein to accommodate the roller.
The deformation of the end of the axle may prevent movement of that end through the aperture away from the roller. However in a preferred embodiment the deformation of the end of the axle prevents movement of that end through the aperture in the body towards the roller (i.e. in the case where a cavity is provided in the body portion, movement towards the cavity is restricted).
In a preferred embodiment the roller has a bore which receives the axle, and the bore has portions of increased diameter at each end to form a pair of corresponding recesses between the axle and the roller. This prevents undue shear force being placed on the axle at the ends of the roller.
There is also provided a downhole device comprising a tubular member, and a fitting according to the first aspect of the present invention secured to the tubular member.
The fitting may be employed in a variety of down hole applications. For instance, the tubular member may comprise a drill string employed in the drilling of the bore hole. In this case the fitting typically reduces friction between the drill string and the wall of a borehole as described in WO 96/34173. Alternatively, the fitting may comprise a centraliser, float shoe or float collar as described in WO 95/21986. In a further alternative, the fitting may be employed in a post-drilling downhole operation, such as NMR well logging.
There is further provided an axle for securing a roller to a fitting for reducing friction in downhole applications, the axle comprising a cylindrical body section having a cavity formed at at least one end which is dimensioned to facilitate deformation of that end of the axle to retain it in position in use.
Both ends of the axle may be provided with such cavities or an enlarged diameter section may be preformed at one end of the axle. The or each cavity may be tapered, for instance frustoconical.
There is further provided a method of securing a roller to a fitting for reducing friction in downhole applications comprising:
positioning a roller so that the ends of the roller are proximate apertures in a body of the fitting; inserting an axle through the apertures in the body and bore in the roller; and deforming at least one end of the axle to prevent that end from moving through the adjacent aperture in the body in use.